Expansible elements such as flat springs, membranes and bellows are broadly used as sensor elements in different automation devices. It is impossible to make a reliable design without modeling the working surface profile of these elements under loading. To get an accurate solution what modeling the expansible elements profile it is necessary to use mathematical tools based on elliptical Jakobi functions. Changing of the vibrating expansible element profile under loade can be characterized by a springy line or the so-called elastics whose equation is represented through the integral of energy, or Euler’s integral, in elliptical functions. As an analog of elastics with bend points a rolling pendulum of variable-length is chosen. A similar solution can be obtained whose changing from a variable-length pendulum to a rocking pendulum under influence of external force varying in time.